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Thread: Tankless WH venting termination location

  1. #1
    DIY Junior Member tjk031's Avatar
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    Default Tankless WH venting termination location

    Hi,

    I'm in the process of deciding to go tankless. I currently have an old low boy gas tank water heater located in my crawl space. I would like to have the tankless heater located in the crawl space as well. I've had a couple of contractors take a look and provide quotes. I'm thinking of a Rannai RU98iN or Rheem rtgh 95dvn. The home is in Roswell GA and I have 2.5 baths.

    I would like to vent to the rear of my house where part of the crawl space wall is above grade.... but there is a number of windows/doors... etc. All contactors took a look and gave different numbers for minimal clearances the vent could be to operatable windows doors. Most manuals, if I'm reading correctly state they need to be 12" away from doors and windows and located above 12" snow level. Does this sound right?? Any advice on the location of the vent termination?

    I do have another wall to the left with no doors or windows. But the grade level is above the crawl space. Any options with venting this route?

    Thanks in advanced for your assistance!!

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  2. #2
    DIY Junior Member tjk031's Avatar
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    I received this reply from the City...

    The clearance specifications should be listed with the manufacturer. Specifically you will need to have a minimum of 12" from any opening and 12" above adjacent grade. There are no restrictions for placement under the deck.

  3. #3
    In the trades Dana's Avatar
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    The clearances in the US differ somewhat from what is required in Canada but you should be able to make this work in GA. See page 7 of the manual.

    Pay close attention to the gas plumbing upgrade requirements- odds are you'll have to bump up to 1-1/4" from whatever was feeding the lowboy, unless the thing is mount VERY close to the meter/regulator. You're now supplying a 199,000BTU/hr burner where a ~40,000BTU/hr burner once stood, and since it's a modulating burner it is more sensitive to pressure changes.

    BTW: A vented crawlspace in a GA climate brings more moisture into the house than it purges, and though enshrined in prior codes and common practices, turn out to be a bad idea for most of the eastern US, particularly in the more humid southeast. (It works just fine in Arizona or California though.) At some point when you're completely out of "honey-do" projects it's worth putting down a ground vapor barrier and a couple inches of rigid EPS on the foundation walls, and foam-sealing/caulking the band joist & foundation sill to make it all nice and air-tight. In an air conditioned house with insulation between the joists you have a pretty signficant mold hazard going on in the crawl space as summertime humidity gets into the cool joist edges & sub-floor. But if you insulate even a little bit at the foundation walls, protect against ground moisture with a sheet of poly or EPDM, the crawl space stays drier year round, and it lowers both the latent cooling load and the heating load of the whole house, and brings any plumbing you have down there at least partially inside the thermal envelope of the house where it won't freeze even during the 100 year cold snap. To meet fire code you'd have to put a thermal barrier over the foam insulation such as half-inch OSB TapConned to the foundation wall 24" o.c. IRC 2012 code min for crawlspace wall-R for your climate (the cool edge of zone 3) is R5, so you could get away with as little as 1.5" of EPS (which runs ~R4/inch). There is a subtle but real comfort uptick when you take the sealed-insulated crawlspace route. A deluxe version would have an inch of EPS on the floor under a 1.5-2" concrete rat-slab, but that's a bit harder to make a rationale for on either a comfort or energy-economics basis.

  4. #4
    DIY Junior Member tjk031's Avatar
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    Thanks Dana! Much appreciated!!

    Yeah I have a vapor barrier down in the crawl space now but no insulation. I haven't had any mold issues yet but would like to seal it off more like you mentioned because in the summer months the Humidity does get mid and upper 50's. Thanks so much for the assistance!

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